Essential plant nutrients include primary nutrients, secondary or macronutrients and trace or micronutrients. Primary nutrients include carbon, hydrogen, oxygen, nitrogen, phosphorus, and potassium. Carbon and oxygen are absorbed from the air, while other nutrients including water (source of hydrogen), nitrogen, phosphorus, and potassium are obtained from the soil. Fertilizers containing sources of nitrogen, phosphorus, and/or potassium are used to supplement soils that are lacking in these nutrients.
According to the conventional fertilizer standards, the chemical makeup or analysis of fertilizers is expressed in percentages (by weight) of the essential primary nutrients nitrogen, phosphorous, and potassium. More specifically, when expressing the fertilizer formula, the first value represents the percent of nitrogen expressed on the elemental basis as “total nitrogen” (N), the second value represents the percent of phosphorous expressed on the oxide basis as “available phosphoric acid” (P2O5), and the third value represents the percent of potassium also expressed on the oxide basis as “available potassium oxide” (K2O), or otherwise known as the expression (N—P2O5—K2O).
Even though the phosphorous and potassium amounts are expressed in their oxide forms, there technically is no P2O5 or K2O in fertilizers. Phosphorus exists most commonly as monocalcium phosphate, but also occurs as other calcium or ammonium phosphates. Potassium is ordinarily in the form of potassium chloride or sulfate. Conversions from the oxide forms of P and K to the elemental expression (N—P—K) can be made using the following formulas:% P=% P2O5×0.437% K=% K2O×0.826%P2O5=% P×2.29% K2O=% K×1.21
In addition to the primary nutrients that are made available to plants via fertilizer added to soil, micronutrients and secondary nutrients are also essential for plant growth. Secondary nutrients include sulfur (S), calcium (Ca) and magnesium (Mg). Micronutrients are required in much smaller amounts than primary or secondary nutrients, and include boron (B), zinc (Zn), manganese (Mn), nickel (Ni), molybdenum (Mo), copper (Cu), iron (Fe) and chlorine (Cl).
The secondary nutrient sulfur is an essential element for plant growth and where deficiency occurs plants show yellowing leaves and stunted growth resulting in crop yield losses. The most cost-effective way to introduce sulfur to soil is to use elemental sulfur, as it is 100% S and therefore has low transport and handling costs. When sulfur is applied to soil in its elemental form (S0), it needs to first be oxidized to its sulfate form by soil microorganisms to enable uptake by the plant. Research has shown that smaller particles oxidize faster when elemental sulfur particles are dispersed through soil, because of their high particle surface area.
To provide the sulfur in a form suitable for application to soil, small sulfur particles cannot be bulk blended with granular fertilizers such as phosphates, nitrates, ureas, and/or potashes to form a physical blend, unless they are incorporated into a pastille or granule with a similar particle size to the rest of the blend. This is because fine elemental sulfur itself presents an explosion/fire hazard when airborne, especially in confined spaces such as spreading equipment, augers, bucket elevators or where there are any potential ignition sources. One other problem with blends containing elemental sulfur particles is that they can undergo size segregation during handling and transportation as the particles settle, resulting in smaller particles (i.e. sulfur) concentrating near the bottom of the bulk blend. Consequently, sulfur is not uniformly distributed throughout the blend, resulting in uneven sulfur dosage when the blend is applied to soil. For example, some treated areas may receive too much sulfur, whereas others may receive too little sulfur.
Sulfur has also been incorporated in fertilizer compositions as a coating, but for a different purpose. Specifically, sulfur has been used in the manufacture of slow release fertilizer compositions as a relatively thick outer coating or shell firmly anchored to the surface of fertilizer particles. In such compositions, the objective is to provide slow release of the underlying fertilizer to the soil, and optionally, for the delivery of sulfur to the soil for subsequent oxidation and plant utilization.
One method for delivering elemental sulfur more uniformly to soil than bulk blending or coating, includes the incorporation of sulfur platelets embedded in the fertilizer portion of the granule, as described in U.S. Pat. No. 6,544,313, entitled “Sulfur-containing fertilizer composition and method for preparing same,” incorporated herein by reference in its entirety. The platelets provide a large surface area, thereby increasing the exposure of the sulfur for oxidation. With these granules, it has been shown that the oxidation rate of elemental sulfur depends on the surface area in contact with the soil after collapse of the granule when soluble compounds diffuse away. As a result, the relative oxidation rate increases with decreasing granule size, and with decreasing percent elemental sulfur in the fertilizer granule.
However, the rate of elemental sulfur oxidation from these granules is still slower than for elemental sulfur dispersed through soil. As illustrated in FIG. 1, the soluble components SOL, such as mono or di-ammonium phosphate (MAP or DAP), triple superphosphate, or urea, of the granule diffuse away, and the insoluble materials including elemental sulfur ES are left in the collapsed granule cavity. This can trap otherwise usable elemental sulfur particles which remain unavailable for oxidation, and therefore unusable to the plant. There remains a need to increase the surface area of elemental sulfur in fertilizer granules to maximize the sulfur available for oxidation within the granule residue.